1. Field of the Invention
The present invention relates to an electric discharge machine and, more particularly, to an electric discharge machine that detects the start of a discharge across the electrode-workpiece gap and then switches from a sub-power supply to a main power supply.
2. Description of the Related Art
It is known that an electric discharge machine for machining a workpiece by wire-cut electric discharge machining or die-sinking electric discharge machining comprises a main power supply and a sub-power supply. The sub-power supply is used to supply the voltage required for starting a discharge across the gap between the electrode and workpiece. Immediately after the discharge has started, the sub-power supply is switched over to the main power supply. In order to control the timing of the switchover, it is necessary to ascertain just when the discharge has started after the gap voltage has been raised. Known means for as certaining the point in time at which the discharge starts include observing that the gap voltage has dropped below a predetermined threshold. The threshold is set so that the corresponding observed voltage level is slightly higher than the level observed during the discharge.
In this type of electric discharge machine, the main power supply and sub-power supply are connected in parallel across the gap between the electrode and the workpiece, the sub-power supply always being used first to apply a voltage across the gap, the sub-power supply then being switched over to the main power supply when a gap voltage below the predetermined threshold level is detected.
FIG. 11 illustrates an exemplary electric discharge machining circuit in a conventional electric discharge machine.
The electric discharge machining circuit has a sub-power supply 1 and main power supply 2 that apply voltages across a gap 20 between a wire electrode (hereinafter referred to as a wire 5) and a workpiece 6, and a detection circuit 30 that detects the voltage across the gap 20.
The sub-power supply 1 applies a voltage for discriminating discharge initiation and more generally the status of the gap 20, while the main power supply 2 supplies the energy for actual machining; voltages are applied in response to the status of the gap 20, e.g., to whether a discharge has started. The detection circuit 30 detects the voltage across the gap 20 to determine the time at which to supply energy from the main power supply 2 after a voltage has been applied by the sub-power supply 1.
The control circuit 10 closes a sub-power supply switching device 3 to apply a voltage from the sub-power supply 1 to the gap 20, and then closes a main power supply switching device 4 in response to a detection signal from the detection circuit 30 to apply a voltage from the main power supply 2 to the gap 20.
In practice, the detection circuit 30 is usually connected to a workpiece mount (hereinafter referred to as a table 19) that secures the workpiece 6 and to a conductor 18 that supplies electricity to the wire 5. The measured voltage is a divided voltage (hereinafter referred to as the detected voltage), being divided by the internal impedance of the sub-power supply 1 and the series impedance of the gap 20, conductor 18, wire 5, workpiece 6 and table 19.
Even when the gap 20 is the same, therefore, the detected voltage varies with the impedance of the workpiece 6.
Energy is supplied from the main power supply upon detecting the voltage drop caused by the discharge across the gap after voltage is applied by the sub-power supply. The voltage drop across the gap is detected by comparing the detected voltage with a predetermined voltage level (hereinafter referred to as the detection level).
Accordingly, if the same detection level is used for machining a workpiece having different impedance, power will be supplied from the main power supply for a workpiece having low impedance, while power will not be supplied from the main power supply for a workpiece having high impedance, thereby making the machining process unstable. Because the-gap voltage cannot be detected correctly, the machining speed becomes unstable; in the worst case, machining becomes impossible.
The appropriate detection level varies with the wire diameter, workpiece material, and other parameters; past practice has been to determine the appropriate detection level by preliminary test machining, and to leave the level unchanged during the subsequent machining process.
Techniques for varying the detection level responsive to the sub-power supply voltage or the machining conditions are disclosed in, for example, Japanese Patent Applications Laid-open Nos. H08-257839 and H07-68418; in these patent documents, however, the detection level is set in advance and is not changed according to the machining state during the machining process.
The detection circuit detects a voltage divided by the internal impedance of the sub-power supply 1 and the series impedance of the gap 20, conductor 18, wire 5, workpiece 6, and table 1. The divided voltage may change during machining according to, for example, the state of the contact between the conductor 18 and wire 5, the impedance of the wire or workpiece, and the internal state of the sub-power supply circuit; when this happens, the time at which power is supplied from the main power supply may be delayed or advanced, making the machining process unstable.
FIG. 12 plots the relation between the detected gap voltage and the width of the gap for a workpiece having high impedance and a workpiece having low impedance. A comparison of these two cases shows that the actual size of the gap differs even if the detected voltages are the same, while the detected voltage differs if the size of the gap is the same. Therefore, the detection level needs to be changed according to the impedance of the workpiece in order to maintain the same gap.
FIG. 13 illustrates exemplary operating waveforms in the conventional art, indicating that the main power pulse frequency differs according to the impedance of the workpiece. In FIG. 13, DL is the detection level, Vgd is the gap voltage detected by the detection circuit 30, S1 and S2 are the operation signals of the sub-power and main power supply switching devices 3 and 4, respectively, C1 is the output signal from the detection circuit 30, and the C2 pulses indicate rising edges of the C1 signal. The number of C2 pulses equals the number of main power pulses, that is, the number of times the main power supply supplies power.
Comparing the number of times the main power supply supplies power during a fixed period Ts in case where the detection level DL is fixed as in the conventional art, the number of C2 pulses increases when the impedance of the workpiece is low and decreases when the impedance is high. This means that the main power pulse frequency increases when the impedance of the workpiece is low and decreases when the impedance is high. Since the main power pulse frequency varies according to the impedance of the workpiece, the machining process becomes unstable.
FIG. 14 shows other exemplary operating waveforms in the conventional art, indicating that the time interval from when a voltage is applied by the sub-power supply until power is supplied from the main power supply varies according to the impedance of the workpiece. In FIG. 14, DL and Vgd are again the detection level and the gap voltage detected by the detection circuit 30. Td1, Td2, and Td3 are time intervals from when voltage is applied by the sub-power supply until power is supplied from the main power supply, for a reference impedance, low impedance, and high impedance, respectively.
Comparing the time interval from when voltage is applied by the sub-power supply until power is supplied from the main power supply in the case where the detection level DL is fixed as in the conventional art, the time interval Td2 for low workpiece impedance is shorter than the time interval Td1 for the reference impedance, and the time interval Td3 for high workpiece impedance is longer than the time interval Td1 for the reference impedance.
Therefore, the time interval from when voltage is applied by the sub-power supply until power is supplied from the main power supply changes according to the impedance of the workpiece, thereby preventing stable machining.
The amount of energy supplied from the main power supply during one discharge is substantially constant, so the total machining energy depends on the number of main power pulses. If, as in the conventional art, a change in impedance leads to changes in the main power pulse frequency or the time interval Td from when voltage is applied by the sub-power supply until power is supplied from the main power supply, the total energy supplied to the workpiece also changes, thereby preventing stable machining.